1. Field of the Invention
The present invention relates to a process for producing an optical material such as a plastic lens, a prism, an optical fiber, a substrate of information recording materials and a filter, and more particularly, to a process for producing a plastic lens of glasses.
2. Description of the Related Arts
Plastic materials have widely been used as various optical materials, particularly as lenses of glasses, because their of light weight, toughness and easiness of tinting. The properties required for optical materials, particularly for lenses of glasses, are a low specific gravity, optical properties such as a large refractive index and a large Abbe number and physical properties such as high heat resistance and high strength. A large refractive index is important to decrease thickness of a lens. A large Abbe number is important to decrease the chromatic aberration of a lens. High heat resistance and high strength are important to facilitate fabrication and also from the standpoint of safety.
As conventional materials having a large refractive index, thermosetting optical materials having a thiourethane structure which are obtained by the reaction of a polythiol compound and a polyisocyanate compound have been proposed (Japanese Patent Publication Heisei 4(1992)-58489 and Japanese Patent Application Laid-Open No. Heisei 5(1993)-148340). Technology to obtain a lens by polymerization of an epoxy resin or an epithio resin with a multi-functional compound have also been proposed in the specifications of Japanese Patent Application Laid-Open No. Heisei 1(1989)-98615, Japanese Patent Application Laid-Open No. Heisei 3(1991)-81320 and International Publication No. WO8910575. Of course, optical materials having further large refractive indices are desirable. A small chromatic aberration is another important property required for an optical material. The larger the Abbe number, the smaller the chromatic aberration. Therefore, a material having a large Abbe number is also desirable. Thus, a material having both a large refractive index and large a Abbe number is desired.
However, the Abbe number tends to decrease with an increase in the refractive index. Plastic materials obtained from conventional compounds have the maximum Abbe number of about 50 to 55 when the refractive index is 1.50 to 1.55, about 40 when the refractive index is 1.60 and about 31 when the refractive index is 1.66. When the refractive index is forced to increase to 1.70, the Abbe number decreases to 30 or less and the obtained material cannot practically be used.
To solve the above problems, the present inventors discovered novel sulfur-containing compounds having an epithio structure from which optical materials having a small thickness and a decreased chromatic aberration, i.e., a refractive index of 1.7 or more and an Abbe number of 35 or more, can be obtained (Japanese Patent Application No. Heisei 8(1996)-214631 and Japanese Patent Application No. Heisei 8(1996)-5797). However, the optical materials described in the above applications do not show a sufficient tint performance and further improvement has been desired. It has been found that an optical material obtained by curing by polymerization of a composition comprising a sulfur-containing compound and a compound having a polar group shows an improved tint performance (Japanese Patent Application No. Heisei 9(1997)-333120). However, this material has a drawback in that components other than the compound having an epithio structure must be added in large amounts to exhibit a further improved tint performance and the refractivity index decreases markedly. For example, the refractivity index decreases to about 1.65 when the tint performance is raised to 75% (the transmittance of light of 25% after tinting). Moreover, a sufficient effect of improvement cannot be obtained in accordance with conventional tinting technologies in which various types of carriers such as aromatic compounds, phenol compounds, alcohols, carboxylic acids, and esters of carboxylic acids and various types of surfactants are used. Thus, a novel technology has been desired in order to tint an optical material having a sufficiently high refractivity index and a sufficiently high Abbe number which is obtained by polymerization of a compound having an epithio structure or a composition comprising the compound.